The field of the invention pertains to processes for manufacturing semiconductor devices such as solar cells.
Typical processes for manufacturing semiconductor devices incorporate the slicing of wafers from an ingot of one conductivity type, at some point diffusing a dopant into the wafer to create portions of the opposite conductivity type and also, at some point, etching unwanted material from the wafers.
For example, a typical process employed in the formation of solar cells begins with the growing of a P-type silicon ingot with no particular attention to its type of crystal orientation, or perhaps with a [100] orientation due to the ease of texturing wafers formed from a crystal with this type of orientation. It is, nevertheless, well known that silicon prefers to grow with the [111] type of orientation and can thus be grown faster with this orientation, and some work has been done on employing such ingots. After the ingot is grown, it is typically centerless ground into a cylindrical shape along which a plane surface is then ground for ease of handling of the wafers which will be formed from the ingot.
The shaped ingot is then conventionally mounted, e.g., on an epoxy base, and sawed into wafers of the order of 0.381 mm (0.015") thickness, with a sawdust loss of approximately the same thickness. A rotating saw or cutting wheel having a periphery charged with diamond particles is typically used and must be replaced approximately every 2000 slices.
Since wafers having a thickness of less than one-third the just-mentioned figure can be readily processed into solar cells, the loss of the semiconductor material during sawing is of great significance from an economic standpoint. In addition, sawing speed limitations, to minimize vibration, breakage and cutting damage, tend to make the sawing somewhat of a bottleneck in the over-all manufacturing process. In attempting to improve upon sawing capabilities, work has been done on the development and use of less traditional types of sawing, including diamond wire sawing (wire charged with diamonds), multi-blade slurry sawing (blades without teeth passed through slurry including, e.g., silicon carbide particles) and multi-wire slurry sawing.
Moving to the post-sawing part of the typical process for forming solar cells, the sawed-off wafers are typically retrieved and placed in multi-wafer cassettes. In these cassettes they can readily be subjected to various liquids in order to dissolve the epoxy mounting, to remove the cutting damage (typically 20% by weight sodium hydroxide), to neutralize the damage-removing solution (typically 10% by weight sulfuric acid) and to cleanse the wafers (e.g., purified water). Then, prior to diffusion to form regions of N-type material (e.g., in an open-tube type furnace), the wafers are typically transferred to another quartz cassette, often with double spacing. The double spacing enables wafers to be diffused with a surface of one against a surface of the other, to eliminate or minimize diffusion into the adjacent surfaces.
Following the diffusion, unwanted material is typically etched away, usually by plasma etching, and contacts are applied to the large surfaces of the wafers. Such etching incudes an edge and front surface etching and, in some processes, also a back surface etching.
The repeated individual handling of wafers in the post-sawing part of the process consumes much time and, along with the use of cassettes, results in undesired wafer damage. Further, the removal of material at various stages, either to shape or eliminate material of an undesired type, results in processing which significantly impinges on the efficiency of the over-all manufacturing process.
The present invention provides a foundation for a number of advances over conventional processes for manufacturing semiconductor devices of the type formed from wafers. Such advances include: higher speed sawing; significantly thinner wafers; significantly less sawing loss; minimal handling of individual wafers; the utilization of ingots with a type of crystal orientation that can be most readily grown; and a telescoping of the processing to improve efficiency.